Rolls such as those employed as roller conveyors for use at high temperature can be made from an inner metal shaft that has an outer insulating cover of asbestos. In manufacturing the roll, the asbestos can be supplied as annular discs, sometimes referred to as "washers", which are assembled on the shaft and pressed together, as discussed, for example, in U.S. Pat. No. 3,802,495. In a technically related teaching in U.S. Pat. No. 3,116,053, there is disclosed a roll having asbestos discs compressed onto a tubular shaft, the asbestos discs alternating between thick and thin discs.
These are pressed together between end walls by a compressive force produced by hydraulic means. Because the binding agent in the asbestos tends to be destroyed at high temperature, it is important to maintain the discs under the axial pressure between the end walls.
A conveyor roll can also be made by sliding a plurality of asbestos millboard annular discs onto a shaft, i.e., annular discs of a major amount of asbestos fiber and a minor amount of binder such as portland cement. These can then be compressed axially on the shaft. As disclosed in U.S. Pat. No. 3,334,010 the asbestos can be a blend of chrysotile plus amphibole asbestos fibers and the binder can be a cement plus clay mixture. In a somewhat technically related disclosure in U.S. Pat. No. 3,456,931 it is discussed that the surface of the roll can be heated to produce an exterior, ceramic surface on the roll.
It has also been proposed to make the discs from ceramic fiber board, having a binder such as portland cement. These discs can be assembled on a metal shaft and compressed into roller form. However, the binder has a tendency to burn out, thus weakening the cover. Such covers have therefore not met with wide acceptance as a suitable replacement for the asbestos millboard covers.
A proposal has also been made to use glassy fibers for rollers, such as rollers used in drawing sheet glass. In this regard, in U.S. Pat. No. 3,763,533 it has been taught to impregnate mineral fiber with an inorganic binder. Strips of felted fiber can be wound around a roller core, impregnated with binder and heated to dry. Such rolls have however not proven to be sufficiently acceptable to find wide use in replacing asbestos rollers.
It has also been proposed to use mineral fibers in feed rollers for roller tunnel kilns. U.S. Pat. No. 4,596,527 teaches the preparation of fibrous tubes, or sleeves, which can be slipped over a feed-roller steel pipe core. These sleeves ostensibly assist in ease of roller maintenance, as fresh sleeves can be readily slipped onto the pipe core during equipment down time after removal of the spent cover layer, which seemingly is required frequently.
However, concerns over the safety of the working environment, as well as over potential damage to conveyed goods from fugitive binder residues at elevated roll use temperature, continues. There is still a need in industry for an improved insulating roll covering, competitive in ruggedness and long-service life with asbestos covers. Such roll cover should also exhibit desirable insulating characteristic, yet provide a more environmentally safe product by elimination of asbestos.